Manual Mode: How to descend

Today I attempted my first descend in manual mode which did not go well. I was about 60m high and in a hover when I switched the controller to manual mode and my phantom dropped like a rock. The phantom appeared to almost free fall, I quickly switched to ATTI mode and hit the throttle, I was barely able to save the quad from a very hard landing. My goal was a fast descend from high elevations to land quickly in situations when the battery is low. How do a properly descend in manual mode with out my phantom dropping like a rock.

I would try it at 60m again but what happened last time scares me to do it at that height the quad was at ground level in seconds pretty scary not very much room for error. When I throttled up after the fall I was definitely in dirty air because it was wobbling violently when it finally began to ascend. The throttle must be very touchy in manual mode. I am assuming you must keep it above 50% the whole time to achieve a nice controlled but fast descend.